Electrical resistance element with a semiconductor overlay

ABSTRACT

A thin film overlay of a semi-conductive material, such as germanium, silicon, indium antimonide, titanium oxide or ferric oxide, covers the resistor track of a film resistance element. A resistance element incorporating this improvement, as compared with one having a noble metal overlay, has a much lower temperature coefficient of resistance. A germanium overlay may be provided by evacuating the space about the resistive element to a pressure of about 2 X 10 5 torr, supplying the germanium material in granular particles ranging in fineness between 180320 mesh by vibration and gravity in a steady and even flow thereof to the evacuated space at a point with respect to which the resistor track is exposed, and then evaporating the germanium at the point by electrical resistance heating to vaporization temperature at about 1,850* C. When used in a potentiometer with a movable contact, a resistance element so made exhibits superior wear stability, and good contact resistance variation characteristics.

United States Patent Healy et al.

[451 June 27, 1972 ELECTRICAL RESISTANCE ELEMENT WITH A SEMICONDUCTOR OVERLAY [72] Inventors: Robert M. Healy, Warrenville, 111.; Robert Marshall, Meguon, Wis.

[73] Assignee: The Bunker-Ramo Corporation, Oak

Brook, lll.

[22] Filed: May 11, 1970 [21] Appl. No.: 36,047

I [52] US. Cl ..338/154, 338/162, 338/308 [51] Int. Cl .,H01c 9/02 [58] Field ofSearch ..1 17/216; 338/98, 162, 190, 338/308, 154, 185, 188, 192, 193

[56] References Cited UNITED STATES PATENTS 2,799,756 7/1957 Graham..... ...338/190 X 3,240,625 3/1966 Collins.... ...338/308 X 3,353,134 11/1967 Elarde..... ...338/308 X 3,368,919 2/1968 Casale ..338/308 X Primary Examiner--bewis H. Myers Assistant Examiner-Gerald P. Tolin Attorney-Frederick M. Arbuckle ABSTRACT A thin film overlay of a semi-conductive material, such as getmanium, silicon, indium antimonide, titanium oxide or ferric oxide, covers the resistor track of a film resistance element. A resistance element incorporating this improvement, as compared with one having a noble metal overlay, has a much lower temperature coefficient of resistance. A germanium overlay may be provided by evacuating the space about the resistive element to a pressure of about 2x10 torr, supplying the gennanium material in granular particles ranging in fineness between 180-320 mesh by vibration and gravity in a steady and even flow thereof to the evacuated space at a point with respect to which the resistor track is exposed, and then evaporating the gennanium at the point by electrical resistance heating to vaporization temperature at about 1,850" C.

When used in a potentiometer with a movable contact, a resistance element so made exhibits superior wear stability, and good contact resistance variation characteristics.

6 Clains, 7 Drawing figures GERMANlUM OVER L'AY FlLM REslsToR 2O SUBSTRATE PATENTEDJUHZ'! 1972 SHEET 10F 3 GERMAMUM OVERLAY F\LM RESISTOR SUBSTRATE TERNHNAL. PA D DEPOS [TED BLANK SUBSTRATE SENHCONDUCYOR OVERLAY DEPOSFFED RESIEFI'OR F\LN\ oe oswau 57- 2c j nvvavroes :QOBERT M. HEALY 8 ROBERT MARSHALL A TFO/QNE Y PATENTEDJunm I972 SHEET 30F 3 MASKING PREHEATING AREAS EVACUATMG RESISTIVE OF TO ELEMENT .RESISTIVE 2X\O"5TQRR TO ELEMENT m 500C SUPPLYING 'SEMICONDUCTIVE EVAPORATlN6 "ggggfil'g MA'EHAL SEMICONDUCTWE THCKNEss \N EvEN FLOWOF MATERIAL B R s N FiNE PARTICLES AT -1850c E ISO-SZOMESH UNMASKNG OF COOLING VENT: N6 REE \ST\\/E TO- TO ELEMENT \55C ATMOSPHERE HAV\N6 sEwcoNDuasvE OVERLAY 1&7. 4

I //Vl/ENTO/5 ROBERT M HEAL) ROBE/87' MARSHALL A FOR/VEY ELECTRICAL RESISTANCE ELEMENT WITH A SEMICONDUCTOR OVERLAY SUMMARY OF THE INVENTION This invention relates generally to electrical resistance devices,.and more particularly to a resistive element therefor having a semi-conductive overlay, and a method of fabricating the same. The element is particularly applicable in variable resistors, such as trimmers, which are useful in affording desired adjustments of electrical circuitry. Generally speaking, adjustment of electrical resistance is accomplished in variable resistor units by movement of a wiper contact along a resistor track. In ,the miniaturized variable resistor units it is preferable that the resistor track be a thin film deposit of material yielding uniform electrical properties. A nickel-chromium-iron alloy has been commonly used for such a deposit, but the alloy is subject to oxidation which gives rise to objectionable wear and noise, in the operation of the variable resistor unit. A partial solution has been afi'orded in the past by coating the resistor track with a thin film overlay of a noble metal such as platinum, rhodium, or palladium, as disclosed in U.S. Pat. No. 3,353,134 issued Nov. 14, 1967 to V. D. Elarde. The noble metal overlays have been limited in application for a variety of reasons. For example, many of them, particularly platinum, are expensive, and the deposition process by thermal evaporation from a tungsten filament is wasteful of material, and gives poor thickness uniformity, as demonstrated by wide divergencies in the properties of various resistive elements deposited in the same batch. Further, the resistivity of the noble metals is usually much lower than that of the resistor track on which they are deposited. For this reason, the thickness of the noble metal overlays must be extremely thin, to limit shunting of the resistor track, i.e. reduction of the terminal-to-terminal resistance value by addition of the overlay. For this reason, thickness has been necessarily limited to less than 100 Angstrom units, at which thickness the resistivity of the noble metal overlay is likely to be between 8,000 and 13,000 ohms per square. In most conformations, such a resistivity practically precludes the use of an overlay with the high value resistors, in the range of 100,000 ohms, and for miniaturized resistors in the medium ranges, of 10,000 to 100,000 ohms. In other words, if the resistivity of the underlying resistor track is higher than 1,000 ohms per square, an overlay as low as 8,000 ohms per square is not practical.

Therefore, to overcome the foregoing and other difficulties in the prior art, a new and improved resistive element is required. In accordance with the present invention a thin film overlay of a semi-conductive material covers the resistor track of a variable resistance element. The semi-conductive material may be germanium, silicon, indium antimonide, titanium oxide or ferric oxide. The fabrication of the resistive elements, in accordance with the present invention, is accomplished in a vacuum deposition apparatus where the individual resistive elements are arrayed in suitable masks to expose their resistor tracks to a heat resistant boat. The array of resistive elements is preheated to a temperature of about 300 C. and subjected to a relatively high vacuum. The semi-conductive material is supplied in granular particles ranging in fineness between 180-320 mesh from a vibratory feeder-hopper down a vertical feed pipe, in an even and steady flow to the boat, which is electrically heated to the vaporization temperature of the semi-conductive material. Evaporation of the semi-conductive material occurs as the particles contact the boat. The thickness of semi-conductive material may be deduced from measurements of the electrical resistance of the deposit upon a standard glass element having terminals and leads to the exterior of the deposition apparatus. After the desired thickness has been reached, the resistive elements are cooled to about 155 C., at which point the vacuum deposition apparatus is vented to the atmosphere and the resistive elements, which then have a semi-conductive overlay in accordance with the present invention, may be removed.

The technological advantages of the invention are best expressed by comparing properties of identically prepared trimmer potentiometer elements having no overlay, and those with germanium and silicon overlays.

One of the properties which is a criterion in evaluating trimmers is contact resistance variation (CRV), often briefly referred to as noise." Military specification MIL-R-22097 prescribes conditions for test. Under these conditions it is found that the noise figure for trimmers with germanium overlay is about one-tenth that of trimmers with no overlay. The figure for those with silicon overlay is'about two-thirds that of trimmers with germanium overlay.

Another property, briefly referred to as wear, is the percentage change in end-to-end resistance, between an initial measurement and one made after 200 cycles of rotation of the movable contact. The change observed in trimmers with either germanium or silicon overlay is less than half that found in those with no overlay.

Temperature coefficient is also of interest, and here comparison must be made between trimmers utilizing a noble metal overlay, as taught in U.S. Pat. No. 3,353,134, and those utilizing the present invention. In general, addition of a noble metal overlay tends to give the composite resistance a strong positive temperature coefficient of resistance. For some applications this may be desirable, but the semi-conductive overlays have a definite advantage where low values of temperature coefficient are desired. The overlay material has a low, even negative, temperature coefficient of resistance which aids in offsetting the normally positive coefficient of the basic resistor track.

Thus, one of the objects of this invention is to provide a novel resistive element for a variable resistor.

It is an object of this invention to provide an element giving reduced contact noise and wear and improved temperature stability in a variable resistor.

Another object of this invention is to provide an overlay which is practical for use on resistor tracks of relatively high resistance values, and of medium resistance values in miniaturized elements.

Still another object is to provide an overlay of semiconductive material for a resistor track in an adjustable resistance device.

Further and other objects, and a more complete understanding of the invention may be had by referring to the following description and claims, taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS For the purpose of illustrating the invention, there is shown in the drawings a form which is presently preferred, it being understood, however, that this invention is not necessarily limited to the precise arrangement and instrumentality there shown.

FIG. I is a perspective view having a partially broken away section, showing a resistive element according to the present invention, and illustrating its application in a potentiometer.

FIGS. 2a through 2d are plan views showing the various steps in depositing of thin film material on a resistive element.

FIG 3 is a schematic perspective view illustrating a vacuum deposition apparatus for fabrication of resistive elements in accordance with the present invention.

FIG. 4 is a schematic block diagram setting forth the steps in fabricating the semi-conductive overlay of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings in detail, wherein like numerals indicate like elements, there is shown diagrammatically enclosed by dotted lines in FIG. 1 a potentiometer 10 which may be substantially the same as that shown in FIG. 3 of U.S. Pat. No. 3,353,134. For simplicity in illustration, some of the elements are omitted or shown diagrammatically, The potentiometer embodies a resistive element 12 formed on a substrate 20 of flat disc configuration as shown, providing a support surface 21. A central opening 22 and a peripheral notch 23 are provided in substrate 20 for mating receiption within a casing (not shown) such as that disclosed in U.S. Pat. No. 3,353,134. The substrate 20 may be fabricated in blank form out of a variety of available insulating materials, such as are afforded by insulating plastics, for example the phenolic and epoxy resins. However, a ceramic such as steatite, alumina, beryllia, glass, quartz and the like are found to be preferable materials for the substrate 20. Borosilicate glass and other high temperature insulating glass are highly satisfactory. The support surface 21 preferably is smooth and regular for bonding of various films of materials thereon, as described hereafter.

A film track 30 of electrically resistive material is bonded on the surface 21 about the periphery of substrate 20, as shown. The material of track 30 may be any of the commonly used resistor materials. For example, a relatively thin film of nickeI-chromium-iron alloy may be provided, having a range of resistance of 500 ohms to 20,000 ohms with a film thickness of 2,000 to I Angstrom units. An alloy composed of to 65 percent nickel, 15 to 75 percent chromium and the remainder iron, has been found to be excellent. Resistance ranges of over 100,000 ohms may be provided, as is well known, by the proper choice of resistance material and by appropriate dimensioning of the film track 30. If desired, cermet materials may be utilized to provide a relatively thick film resistance element for the track 30, having a thickness appreciably above 2,000 Angstrom units. Thus a relatively wide range of resistance values, say from 10 ohms to 10 meg ohms, may be provided in different versions of variable resistor units. The track 30 may be bonded on to the surface 21 by the usual techniques presently common in the art.

Terminal means are provided by the deposit of metallic terminal pads 40, 42 at opposite ends of the track 30, bordering the peripheral notch 23. Preferably, of course, pads 40, 42 are a good electrical conductor such as nickel, silver, etc., having a good electrical contact with the track 30.

Electrical leads 24 and 25 are bonded to the terminals 40, 42 and extend to the potentiometer end terminals 26, 27. A wiper or slider contact, a portion of which is shown at 60, is connected by a lead 28 to the potentiometer intermediate terminal 29.

An overlay film 50 of semi-conductive material coats the track 30 as illustrated. The semi-conductive material of overlay 50 is selected from a group consisting of germanium, silicon, indium antimonide, titanium oxide and ferric oxide. The preferred materials are germanium and silicon. The thickness of the overlay 50 is not critical, as was the case in the noble metal overlays of the prior art, since the resistivity of the semiconductive material is considerably higher than that of the resistor track 30. Preferably, thickness of the overlay 50 is between 100 and 4,000 Angstrom units.

FIGS. 2a through 2d illustrate the fabrication of a resistive element in accordance with the present invention. A blank substrate 20 is shown in FIG. 2a. The terminal pads 40, 42 are first deposited thereon as illustrated in FIG. 2b. Then resistor track 30 is provided on the surface 21 as illustrated in FIG. C. And finally, as shown in FIG. 2d the semi-conductor overlay 50 is provided over the resistor track 30 to provide a resistive element in accordance with the present invention.

A flash evaporation apparatus for the fabrication of resistive elements in accordance with the present invention is indicated generally by the numeral 80 in FIG. 3. Basically, the vacuum deposition apparatus 80 consists of a solid base 81 and bell jar 82 enclosing a space which may be evacuated by the vacuum pump 83. The apparatus 80 may be vented to the atmosphere through a vent 84. Support rods 85 are provided to position various elements within the bell jar 82.

A mask array dome 90 is supported on the support rod 85 a distance above the base 81 and provides a parabolic or spherical support orientation for the removable trays 91 with respect to a point within the bell jar 82. Depressions 92 in the trays 91 are shaped for reception of the substrate blanks 20 shown in FIG. 2a. The depressions 92 include openings (not shown) therethrough corresponding to the resistor track 30 of each resistive element 10. It is understood that preferably the shape of the depressions 92 accounts for the peripheral notch 23, so that the resistor tracks 30 may be properly oriented with respect to the openings (not shown) in depressions 92.

A heater dome 95 is spaced above the mask array dome on the support rods 85, as shown. Heater elements 96, indicated by the dashed Iines, are provided on the underside of the heater dome above each removable tray 91. A power source 97 is connected .by leads 98, 99 to each heater element 96, and in this manner, provides energy to heat the resistor trays 91 facing the inside of the heater dome 95. It is understood that the heater element 96, power source 97, and leads 98, 99 shown in FIG. 3 are merely simplified schematic versions, and far more sophisticated means may be visualized by those skilled in the art.

A vibratory feeder-hopper 100 extends from a support rod outwardly over theheater dome 95. The vibratory feederhopper may be of the well known type energized by electrical solenoids or motors to shake with regular vibrations. A vertical feed pipe 102 having a funnel 103 at the top end extends through openings at the center of the mask array dome 90 and heater domer 95. The funnel 103 engages within the opening through heater dome 95 to support the feed pipe 102 in a ver-. tical disposition, as shown. The funnel 103 and pipe 102 extends freely through opening 104 in the mask array dome 90. The funnel in 103 is positioned directly beneath the vibratory feeder-hopper 100.

A boat of a highly heat resistant material such as tantalum or molybdenum is provided at a central point with respect to the trays 91 in the mask array dome 90. Preferably, the boat is positioned at a point which is the focus of the parabolic or spherical support orientation of the trays 9I. Resistive heaters 106, 107 are connected by power lines 108, 109 to a source of power 110, and serve to heat the boat 105.

A bracket extends from a support rod 85 as a mounting for a glass plate 116. Spaced electrodes 119 are provided on the glass plate 116 and leads 117, 118 extend therefrom outside the bell jar to a meter 120 for readings of resistance across the glass plate 116 between electrodes 119. The arrangement outlined is commonly referred to as a glass monitor," and its use is described at a later point in this specification.

The steps in the process of depositing the semi-conductive overlay 50 on a resistor track 30 are now described with reference to the apparatus shown in FIG. 3 and the block diagram shown in FIG. 4. The individual resistor elements 10, each having a resistor track 30 such as that shown in FIG. 2c, are arrayed in the movable trays 91 by placing them in the depressions 92 with their surfaces 21 face downward. Only the tracks 30 are exposed through openings in the depressions 92, the remainder of surfaces 21 being masked therein. The trays are placed in the mask array dome 90 with the resistor tracks 30 exposed to a focus point at the tantalum boat 105. The space within the bell jar 82 is evacuated by the vacuum pump 83 to a pressure of about 2X 10' torr. Current is supplied from the source 97 through leads 98, 99 to provide heat at heater elements 96 until the array of masked resistive elements are preheated to a temperature of about 300 C. A supply of semiconductive material, such as germanium, is provided in the vibratory feeder-hopper 100 in a granular form having particle fineness ranging between 180-320 mesh. Vibration and gravity provide an even flow of material. Particles of the semiconductive material flow from the feeder-hopper 100 and down the feed pipe 102 to drop on to the tantalum boat 105. The heaters 106, 107 are energized through power lines 108, 109 by the source of power 110. The tantalum boat 105 is heated to a temperature of about l,850 C. At this temperature, the semi-conductive particles falling onto the boat are almost instantly vaporized, so that the process is characterized as flash evaporation." The vapor disperses in the direction of the arrows and the result is a deposit on the surfaces of the resistor tracks 30.

At the same time, a coating of the semi-conductive material will be deposited upon the plate 116, and with increasing thickness of the deposit, the resistance between the electrodes 119, as observed on the resistance measuring instrument 120, will decrease. The glass monitor thus provides a means of determining the end point of the process, i.e., the point at which the desired thickness has been deposited on the tracks 30. Preferably, thickness of the overlay on the tracks is between 100 and 4,000 Angstrom units. A typical deposit of germanium would be one having a room temperature resistivity of 500 megohms per square. At 300 C. this would be approximately 190,000 ohms per square. Such a deposit can be obtained in about 2% minutes.

When the desired thickness is reached, the vibratory feederhopper 100, heat elements 96, 97 and heaters 106, 107 are turned off and the array is allowed to cool to about 155 C. Then the bell jar 82 may be vented to the atmosphere by opening vent 84. The individual resistive elements may be removed from the apparatus 80 and will have a semi-conductive overlay in accordance with the present invention.

While a particular embodiment of the invention has been illustrated and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects. Deposit of the overlay by flash evaporation has been described, but suitable coatings within the purview of the invention can be laid down by sputtering, thermal evaporation, electron beam evaporation, deposition from plasma, etc. It is therefore the aim in appendant claims to cover all such changes and modifications as followed in the true spirit and scope of the invention.

We claim:

1. A potentiometer comprising:

- a resistance element which includes an insulating substrate providing a support surface,

- an elongated film track of electrically resistive material bonded on said surface,

- terminals at each end of said track for connecting to electric conductors, and

- an overlay film of homogenous non-particulate semi-conductive material coating said track, said material being selected from a group of semi-conductive materials such as germanium, silicon, indium antimonide, titanium oxide, ferric oxide or the like, and

- an electrically conductive slider making contact with said film and movable along the length thereof, said slider having an electrically conductive connection therefrom to a third terminal.

2. A potentiometer in accordance with claim 1 wherein the resistivity of -said film track is in excess of 1,000 ohms per square.

3. The potentiometer of claim 1 wherein said resistive material is an alloy consisting essentially of nickel, chromium and iron.

4. The potentiometer of claim 1 wherein said electrically resistive material is an alloy consisting essentially of 20 to 65 percent nickel, 15 to 75 percent chromium and the remainder iron, said track having a thickness of between and 4,000 Angstrom units.

5. The potentiometer of claim 1 wherein said overlay film has a thickness of between 100 and 4,000 Angstrom units.

6. The potentiometer of claim 1 wherein said overlay film has a thickness such that the room temperature resistivity thereof is in excess of one hundred megohms per square. 

2. A potentiometer in accordance with claim 1 wherein the resistivity of said film track is in excess of 1,000 ohms per square.
 3. The potentiometer of claim 1 wherein said resistive material is an alloy consisting essentially of nickel, chromium and iron.
 4. The potentiometer of claim 1 wherein said electrically resistive material is an alloy consisting essentially of 20 to 65 percent nickel, 15 to 75 percent chromium and the remainder iron, said track having a thickness of between 100 and 4,000 Angstrom units.
 5. The potentiometer of claim 1 wherein said overlay film has a thickness of between 100 and 4,000 Angstrom units.
 6. The potentiometer of claim 1 wherein said overlay film has a thickness such that the room temperature resistivity thereof is in excess of one hundred megohms per square. 